parallel-tests: Fix driver for nonempty executable extension.

parallel-tests: Fix driver for nonempty executable extension.

* automake.in (handle_tests): New substitution `%am__EXEEXT%',
defined as 'FALSE' for non-generic rules, or if no programs are
built at all.  Otherwise, define it as configure conditional.
* lib/am/check2.am [%am__EXEEXT%] (%EXT%$(EXEEXT).log): New
conditional generic rule.
* m4/init.m4: Hook an m4_provide of `_AM_COMPILER_EXEEXT' onto
Autoconf's `_AC_COMPILER_EXEEXT' macro.
(AM_INIT_AUTOMAKE): If `_AM_COMPILER_EXEEXT' has been provided
at `AC_CONFIG_COMMANDS_PRE' time, then introduce a conditional
`am__EXEEXT', defined to true iff `$EXEEXT' is nonempty.
* tests/check5.test: Only match `_EXEEXT_[1-9]' here, to avoid
false positives stemming from `@am__EXEEXT_TRUE@'.
* NEWS: Update.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>